The present invention relates to electric resistance heating elements. More particularly, the present invention relates to electric resistance heating elements for heating fluids such as gasses or liquids, where the heating element has an overmolded thermally conductive polymer coating, which includes integrally molded surface enhancements to increase the thermal transfer efficiency between the assembly and the fluid to be heated.
Electric resistance heating elements, such as those used in connection with water heaters, have traditionally been made from metallic and ceramic components. A typical prior art construction, for example, includes a pair of terminal pins brazed to the ends of a wound metallic coil element, which is then inserted axially through a U-shaped tubular metal sheath. The resistance coil is electrically insulated from the walls of the metal sheath by a powdered ceramic material, such as magnesium oxide. The entire assembly is then placed through an opening in the wall of a tank and sealed in place, leaving the U-shaped tube exposed on the interior of the tank to heat the fluid contained within the tank.
While such conventional heating elements have been commonly used as the primary heating element in the water heater industry for decades, there are a number of widely recognized deficiencies with this type of heating element. For example, because the entire system is generally surrounded by water during operation, galvanic currents occurring between the metal sheath of the element and any exposed metal surfaces in the tank can cause corrosion of the various anodic metal components of the system. The metal sheath of the heating element, which is typically formed from copper or copper alloy, also attracts lime deposits from the water, causing them to build up on the surface of the sheath, further contributing to premature failure of the heating element. Additionally, the use of brass and copper fittings and tubing has become increasingly more expensive as the price of copper has increased over the years.
In an attempt to overcome the deficiencies associated with the metal heating elements, a number of electric heating elements that include a plastic outer sheath have been developed in the prior art. In one application, the metallic tube sheath is simply replaced with a plastic sheath, which is used in conjunction with a conventional wound resistance wire coil and powdered magnesium oxide coating. Since the plastic sheath is non-conductive, there is no galvanic cell created between the element and the other metallic parts of the heating unit in contact with the water in the tank. Also, the use of a plastic sheath eliminates the possibility of lime buildup. Unfortunately, for various reasons, the prior art plastic-sheath heating elements are not capable of attaining sufficiently high wattage ratings over a normal useful service life and as a result, are not widely accepted.
Another prior art heating element includes a hollow tubular core around which a resistance heating element is wound. A thermally conductive polymer coating is then placed over the resistance element and tubular core to form a hermetic seal around the resistance wire. While this type of heating element can produce and withstand the required operational wattage range to be commercially acceptable, they also have several drawbacks. To enhance the transfer of heat to the fluid medium, the resistance heating element is formed as a hollow tube. This hollow tube design has the tendency, however, to trap pockets of air on the interior cavity of the tube that lead to overheating, burnout and eventual element failure. Further, the surface of the element that contacts the fluid medium is smooth, providing a reduced surface area for heat exchange between the heating element and the fluid medium. Finally, because the heating element is subjected to significant temperature changes in its normal range of operation, the material is subjected to a great deal of thermal stress in the form of thermal expansion. Since the polymer coating has a coefficient of thermal expansion (CTE) that is different than the CTE of the base tube and the resistance heating element, the various materials tend to expand and contract at different rates. This differential expansion produces mechanical stresses in the polymer coating that may result in cracking of the coating, thus allowing the fluid to leak into the element, which ultimately results in the failure of the element.
Another environment in which this type of heating element is encountered is in heating aquariums. In the prior art, aquarium heaters are manufactured as described above with a wound resistance heating element supported on a ceramic core, however, the outer casing is typically made from glass. The use of glass is necessary to insure that in the event that the heating element becomes electrically shorted, no electrical current will flow into the aquarium creating a potential hazard to any person encountering the aquarium. While in this case the glass provides an effective enclosure for the heating element, it is also relatively fragile and subject to breakage.
The present invention provides for an electric heater element that is injection overmolded with a thermally conductive polymer having a thermal conductivity of at least 3 W/mxc2x0 K. In addition, the heater element of the present invention includes surface enhancements to increase the overall outer surface area of the element that is in contact with the fluid thereby enhancing the efficiency of heat transfer into the fluid.
In this regard, the instant invention includes a solid core heater element that can be used to replace the conventional heater elements described above while overcoming many of the deficiencies noted with respect to the prior art elements. Since the core of the heater element is solid, it does not have any voids that could potentially trap pockets of air that would cause hot spots on the surface of the heater element and eventual element failure. The solid core of the element is molded having channels in its outer surface where the resistance heating element is wound and supported. The resistance heating element is spirally wound starting at the base of the core in a circular fashion to the end of the core and is threaded back down a passage in the center of the core. In this manner, the resistance heating wire can be connected to two metallic pins at the base of the core to facilitate electrical connections once the element is completed and installed in a finished product.
Once the resistance heating element is wound around the core, the outer covering is injection overmolded onto the core sealing the heating element and shielding it from the exterior environment. While the outer covering seals the heating element, it also includes surface enhancements such as concentric fins, pins or discs that increase the contact surface area of the outer cover, further enhancing the heat transfer properties of the heating element. The outer cover is injection molded from a thermally conductive polymer material that includes a base polymer matrix and a thermally conductive filler loaded therein. Further, the polymer base matrix is selected in such a manner to match the CTE of the base material to the CTE of the rest of heating element to prevent CTE mismatches and the resulting cracking of the outer covering. After the overmolding is complete, the assembly is inserted into a base support to facilitate integration with the finished product such as a water heater.
Finally, it can be appreciated that the present invention also includes the method of manufacturing the present invention in accordance with the disclosure contained herein. The method of manufacture includes providing a central core element around which a resistance heating element is wound, injection overmolding a thermally conductive polymer coating over the core element and installing the assembly into a support base.
Therefore, it is an object of the present invention to provide a heating element that is hermetically sealed for use in heating fluids. It is a further object of the present invention to provide a heating element with an outer casing that includes integral elements for enhancing the surface area and thermal transfer efficiency of the heating element. It is yet another object of the present invention to provide a heating element with an outer casing that is resistant to cracking as a result of thermal expansion of the inner core or breaking due to droppage or rough handling.